I. Field of the Invention
This invention relates to chemical compositions having utility as chelating agents and particularly to biodegradable chelating agents. This invention particularly relates to chelating agents having utility in agriculture and particularly in fertilizers.
II. Description of Relevant Art
In agriculture, metal ions are essential nutrients for plant growth, development, and disease resistance. Plant nutrient insufficiencies, because of the unavailability or exhaustion of metal ions, are very often the cause of poor plant growth and development. Crop deficiencies occur at extremely low levels of metal deficiency, that is to say, at levels of parts/million in the plant tissue.
Both soil and foliar application of chelated metal ions may prevent, correct or minimize crop mineral deficiencies. Chelated complexes have been favored because the chelated metal ions remain soluble in different or changing environments. Conventional products have used synthetic chelates. However, even though widely accepted as the best method for the administration of metal ions, synthetic chelates persist in the environment. Therefore, less persistent, yet still efficient, chelating systems have been sought.
The use of naturally occurring organic acids, and their derivatives, as chelating agents to provide an inexpensive and biodegradable alternative has been proposed. However metal chelates of citric acid were reported as unstable at a pH above 7 and as having inferior properties to the synthetic chelating agents.
Because some synthetic chelates are not biodegradable, the uses of such compounds are being regulated in many parts of the world. Often the use of naturally occurring organic acids and organic bases that readily degrade are not always efficacious. The use of a synthetic chelate that biodegrades may substitute for the synthetic compound with environmental persistence and still be effective. The present invention solves those needs.
A variety of methods for preparing polysuccinimide with subsequent hydrolysis to polyaspartic acid (or salts) have been described in the literature and patents. In addition uses of those compounds as chelates have also been reported.
U.S. Pat. No. 4,590,260 to Harada et al. teaches that copoly (amino acids) are produced by heating a mixture of at least one amino acid with at least one of ammonium malate, ammonium maleate or ammonium fumarate, or ammonium salts of malic, maleic or fumaric acid monoamide, or malic, maleic or fumaric acid monoamide or diamide, and hydrolyzing the reaction mixture under neutral or alkaline condition. The method is said to be simple and easy to handle, and therefore, suitable for industrial applications.
U.S. Pat. No. 5,362,412 to Hartman, et al., teaches use of iminodisuccinic as a nonphosphorus-containing biodegradable stabilizer, and U.S. Pat. No. 5,468,838 to Boehmke; teaches a process for the preparation of polysuccinimide, polyaspartic acid and their salts, where, polysuccinimide, polyaspartic acid and their salts are prepared by reaction of maleic anhydride and ammonia, polycondensation of the resulting product in the presence of a solubilizing agent and, if appropriate, hydrolysis.
Patent Application Publication WO9845251A1 of GROTH, et al., entitled “Preparation and Use of Iminodisuccinic Acid Salts,” teaches that iminodisuccinic acid alkaline salts can be prepared by reacting maleic acid anhydride (MAA), alkali metal hydroxide NH3 and water in a molar ratio of MAA: alkali metal hydroxide: NH3: H2O=2:0.1–4: 1.1–6.5: 5–30 at 70–170° C. and 1–80 bar for 0.1–100 hours. The reaction mixture is mixed with additional H2O and optionally alkali metal hydroxide and is freed distillatively of NH3 at 50–170° C. and 0.1–50 bar and then set at a 0.1–50 bar and then set at a solids content of 5–60 weight % using H2O. The iminodisuccinic acid alkaline salts are said to be useful for increasing the brightness and brilliance of plant fibres in paper manufacture.
Patent Application Publication No. JP8012631, of Yamamoto Hiroshi also teaches a procedure for production of iminodisuccinic acid and it's alkali metal salt and a biodegradable chelating agent containing the same. In this procedure, a tetraalkali metal salt of iminodisuccinic acid is obtained by adding a half ester of maleic acid to aspartic acid or ammonia under an alkaline condition followed by hydrolysis and evaporation to dryness. A second objective iminosuccinic acid is obtained by the above addition reaction followed by hydrolysis and then addition of sulfuric acid (without conducting an evaporation to dryness). In these processes, use of L-aspartic acid in place of the ordinary aspartic acid is said to produce D,D-form-free iminodisuccinic acid and a tetraalkali metal salt thereof. Alternatively, L,L-iminodisuccinic acid is said to be selectively obtained by prior crystallization and/or washing of a mixture of the L,L-form and D,D-form of iminodisuccinic acid or a tetraalkali metal salt thereof. The other “objective biodegradable chelating agent” is said to contain, as the active ingredient, the D,D-form-free iminodisuccinic acid and/or an alkali metal salt thereof.
U.S. Pat. No. 4,839,461 to Boehmke describes a procedure for preparation of polyaspartic acid from maleic anhydride, water and ammonia. In the procedure, maleic anhydride is converted into a monoammonium salt in an aqueous medium with addition of concentrated ammonia solution. The water must be evaporated out of the aqueous solution, and the monoammonium salt is subjected to polycondensation to give poly succinic imide in the melt at temperatures of, for example, 125 degrees to 140 degrees Centigrade. Viscous phases which are difficult to control industrially are passed through during this procedure. In the course of the condensation, thermal insulation may occur, which severely delays heat transfer to end the reaction. Suitable apparatuses for detaching the wall layers and thorough mixing are proposed in the specification. For subsequent neutralization for preparation of salts, the mixture must again be converted into the liquid phase. This solution must be evaporated again for preparation of the solid salts
A series of patents to Koskan et al., U.S. Pat. No. 5,057,597; U.S. Pat. No. 5,116,513; U.S. Pat. No. 5,152,902 and U.S. Pat. No. 5,221,733, teach methods for thermal polymerization of aspartic acid in a fluidized bed to form polysuccinimide which is then hydrolyzed to polyaspartic acid (sodium salt) using sodium hydroxide. Uses of polyaspartic acid as calcium carbonate, calcium and barium sulfate and calcium phosphate scale inhibitors are also described in these patents.
U.S. Pat. No. 5,219,952 and U.S. Pat. No. 5,296,578 to Koskan & Meah teach production of polysuccinimide and polyaspartic acid (and salts) from maleic anhydride, water and aqueous ammonia. Polysuccinimide is said to be produced in at least 90% of theoretical yield by heating the maleic anhydride, water, ammonia mixture at 220 degrees–260 degrees Centigrade.
U.S. Pat. No. 4,696,981 to Harada & Shimoyama teaches preparation of polysuccinimide from precursors of aspartic acid such as monoammonium, diammonium, monoamide, diamide and monoamideammonium salts of malic, maleic and fumaric acid and mixtures of these materials by irradiating them with microwaves. The resulting polysuccinimide is hydrolyzed to form polyaspartic acid. Similarly mixtures of at least one amino acid and precursors of aspartic acid are taught to be irradiated with microwaves followed by hydrolysis to produce copolyamino acids of aspartic acid.
U.S. Pat. No. 5,286,810 to Wood discloses the preparation of higher molecular weight copolymers of polyaspartic acid said to be suitable for the inhibition of scale deposition by reacting maleic acid and ammonia in stoichiometric excess with a diamine or a triamine at 120 degrees–350 degrees Centigrade. The resulting copolymers of polysuccinimide are said to be converted to a salt of the copolymer of polyaspartic acid by hydrolysis with a hydroxide.
U.S. Pat. No. 5,292,858 to Wood teaches copolymers of polyaspartic acid prepared by making maleic half esters followed by addition of an equivalent of ammonia and an amine and heating to 120 degrees–350 degrees Centigrade. When an equivalent of alcohol is distilled off, a copolymer of polysuccinimide is said to be formed, which is hydrolyzed with hydroxides to form amide copolymers of polyaspartic acid.
U.S. Pat. No. 5,763,634 to St. George, et al. teaches a process for preparing ferric chelate solutions of alkali metal polyamino succinic acids.
Notwithstanding these various known procedures, prior art systems involving succinic acids, when used for chelation have failed to achieve their assumed bonding potential, rendering prior art compounds less attractive as chelating agents in the fertilizer market place. A reference by T. N. Polynova, L. A. Zassourskaya and M. A. Porai-Koshits, entitled, “Crystal Structures of d-Transition Metal Complexes with Iminodisuccinic Acid,” published by the Chemical Department, Moscow State University, Moscow, 119899, Russia discusses the problem.
This Poynova et al. reference teaches that “in complexation with d-transition metals, the ligand-iminodisuccinic acid (H4ids) does not realize all its coordination possibilities in any of the complexes studied by X-ray analysis. Potentially, the H4ids ligand is pentadentate, but in compounds [Co(H2O)6][Coids(en)]2. 4H2O (I) and [ZnH2ids(H2O)2] (II) ids4- and H2ids2-ligands are tetradentate regardless of the differences in aprotonization, stoichiometric composition and valent state of complexing atoms. The coordination of Co(III) and Zn(II) in the form of a distorted octahedron is made up of the N-atom and three O— atoms of the H4ids ligand as well as of two N-atoms of the en ligand or two water molecules in I and II respectively. Hence, the three metallocycles are formed as one [[beta]]-carboxyl branch [which] remains uncoordinated by the metal (aprotonized in I and protonized in II). In I an intramolecular H-bond is formed between the free [[beta]]-carboxyl and the amino group of ethylene-diamine. In II the intramolecular H-bond is not formed: H-atoms (one of them connected with N, another, with the O-atom of the uncoordinated [[beta]]-carboxyl group) form intermolecular H-bonds”.
Thus, the prior art teaches that while the Iminodisuccinates have value, they fail to provide adequate chelation of metal ions, particularly for uses such as in phosphate fertilizer solutions. Such failure stems from several qualities of the compound. The most efficacious of chelation compounds have at least six nonbonded electron pairs; and most mineral ions share a coordination number of six. Because some isomers of Iminodisuccinate are tetradente, the complexes are vulnerable to carbonate, hydroxide, and, phosphate participation in the complex and such complexes are insoluble. Further, iminodisuccinic isomers have the ability to donate four electron pairs, and with the prior art methods of synthesis, there are a number of isomers that can not provide five pairs because of bond strain or bond angle limitations. Iminodisuccinate is mostly found as a tetradente-chelating agent. Being tetra dente prevents the compound from being suitable as a commercial chelating agent for use in agriculture and industry, and particularly for example in phosphorus fertilizers.
Accordingly, there has been a long felt, and unfulfilled need for more efficient, more economical, and more environmentally friendly chelation methods and compositions. These methods and compositions could be used to deliver micronutrient levels of trace metals to plants, to aid in delivery of other horticultural and agricultural chemicals, and, for fertilizers with required metal nutrients necessary for plant growth, development, and disease resistance. The present invention meets such needs.